18.4.1 Examples of Freestanding L, C, R Loudspeakers
These are the loudspeakers commonly used as left and right front loudspeakers, and occasionally as center loudspeakers as well. Some are small bookshelf-sized products, needing a subwoofer to complete the spectrum, and others are full-range floor-standing units that may need no such help.
In Figure 18.15 we start with examples of poor loudspeakers, or loudspeakers that fail to live up to expensive expectations. That there are loudspeakers with problems is no surprise. That they are rewarded with excellent reviews is harder to swallow. The evidence of many years of experience is that all these reviewers, in an unbiased listening situation, would very likely have recognized the problems for what they were. Real measurements would have settled any debates, and the manufacturer would, in a fair world, get a reminder to pay closer attention to the first task of a loudspeaker: to be an accurate reproducer. Instead, the message is perpetuated that just about anything goes. The fact that truly good loudspeakers also get favorable reviews (as was exemplified in Figure 18.14) indicates that the existing reviewing process has a large tolerance range.
FIGURE 18.15 (a) An attractive-looking floor-standing unit from an internationally highly-regarded brand. The sound is not good. (It is what I call an "ecologically irresponsible" design, since it is truly a waste of raw materials.) If an engineer was involved, one has to suspect that the design was "phoned in." The cynicism of this brand is made all the more striking because the parent company has competent loudspeaker design capabilities. This product was farmed out to a local supplier to meet a "marketing" need in another country. (The world apparently needed another example of bad sound.) The loudspeaker in (b) is ten times more expensive but unfortunately not ten times better sounding. It is evident that this combination of an electrostatic mid-high-frequency unit and a conventional woofer does not go far enough in the sound-quality department. The DI of around 5 dB is about what might be expected of a dipole radiator (4.8 dB), but the frequency response seriously sags over the top two octaves, and the crossover around 100 Hz to the "one-note" bass unit could be improved. (c) An expensive two-way cone/dome bookshelf with "exotic" ingredients. However, any virtues they may have contributed are swamped by frequency-response and directivity problems. It had no low-bass output, but the excessive upper bass made it sound notably flat, dull, and moderately colored. It got flattering reviews and was the chosen "reference" speaker of one magazine editor. (d) A very expensive product claiming the advantages of exotic diaphragm materials. At this price, one would have reason to expect something better than this performance, which exhibits both frequency-response and directivity anomalies. There also appears to be a significant resonance around 4 kHz, a bump that is present in all of the curves. The reviews have been fulsome.
One could go on exploring the infinite ways of failing to be good, but instead let us look at what is being done by other manufacturers at affordable prices (see Figure 18.16). At low prices there are no fancy hand-polished wood finishes, no high-toned industrial designs, and the ability to play very loud may have been sacrificed, but in terms of sound quality, these are all excellent loudspeakers. Add a subwoofer to any of the bookshelf designs shown in Figures 18.16a, (b), and (c), and set the surround processor to "small," engaging an electronic high-pass filter, and the combination is capable of delivering a truly high-end, high-sound-level performance.
FIGURE 18.16 (a), (b), and (c) Examples of three good bookshelf loudspeakers. The high price of (c) is explained by its elegant visual presentation, materials, and high-power drivers. (d) An example of a very well-balanced design; some may call it the "point of diminishing returns" in that it has the bandwidth and sound quality to be very rewarding.
Loudspeaker (a) is a remarkable performer at the price ($200/pair during sales) and sets a standard for sound quality that is hard to improve on at any price. A first-time buyer is off to a good start. At almost seven times the price, the bookshelf loudspeaker (c) has slightly smoother curves, features significantly better bass, can play much louder, and has an elegant appearance (in other words, pay more, get more). The full-range floorstanding loudspeaker in (d) needs no subwoofer, and it achieves its rather excellent performance-per-dollar status by being well engineered and visually inert: a simple-to-manufacture tall, rectangular box in simple vinyl woodgrain wrapping.
Figure 18.17 shows samples of two excellent, high-priced loudspeakers, that do almost everything well. To these should be added loudspeakers "R" and "I" in Figure 18.14. Collectively, these are examples of the present-day "kings of the hill." There are others, of course, but the measurements do not look very different. When they are put against each other in double-blind tests, the audible differences are small, somewhat program dependent, and listener ratings tend to vary slightly and randomly around a high number. In the end there may be no absolute winner that is revealed with any statistical confidence; the differences in opinion are of the same size as those that could occur by chance.
FIGURE 18.17 (a) A high-priced, high-power floorstanding cone and dome loudspeaker. (b) A high-priced, high-power floorstanding cone woofer combined with compression-drivers and horns for the mids and highs. (a) has wider dispersion (lower directivity index) than (b), a characteristic difference between cone/dome and cone/horn designs.
18.4.2 Horizontal Center-Channel Loudspeakers
A direct-view video display is a challenge for the center loudspeaker. Some people give up in frustration and use the stereo "phantom" center. DON'T DO IT! (See Section 9.1.3.)
Most people use a horizontal loudspeaker system commonly configured in one of the two options shown in Figure 18.18. The simple one, often called the "midrange-tweeter-midrange" or MTM, arrangement is usually found in entry-level products but also, occasionally, in some expensive products. In its basic configuration of both woofers operating in parallel, crossing over to a tweeter - a two-way design - it is not optimum because of off-axis acoustical interference. In Figure 18.18a it is seen that this interference is symmetrical, so both lateral reflections suffer from the same flaw, affecting sound quality.
FIGURE 18.18 Two common configurations for horizontal center-channel loudspeakers. (a) The MTM design has two "midranges," which actually are woofer/midranges, that acoustically interfere with each other at increasing horizontal angles. This is because they are physically separated, and both radiate sound up to high frequencies to cross over to the tweeter. This can be seen in the polar plot displaying a horizontal view of sound level as a function of frequency and angle. Moving radially from the center, the concentric rings represent 200 Hz, 500 Hz, 1 kHz, 2 kHz, 5 kHz, 10 kHz, and 20 kHz. The areas in white represent sound levels within 3 dB of the axial (0°) output. The contours at the onsets of progressively darker gray areas represent sound levels at 6, 12, 24, and 48 dB below the axial output. It can be seen that moving horizontally off axis, the first interference dip in the frequency response appears at under 10° off axis. Recall that in Figures 16.6 and 16.10, it was shown that a horizontal dispersion of ±30° was required for the center loudspeaker to deliver intact direct sound to all listeners in a typical home theater. This figure shows that by 30° this loudspeaker is experiencing heavy acoustical interference, and the output has dropped seriously over a wide frequency range. This is not good. (b) depicts a much better design, in which a midrange loudspeaker has been added in a central location. This allows the widely spaced woofers to be turned off at a lower frequency, and the interference dip disappears. The superior dispersion of the small midrange driver is apparent. The result is that at 30° the sound is unimpaired; only a normal slight loss above 10 kHz due to tweeter directivity is seen (this would be seen in any loudspeaker with a conventional dome tweeter). In fact, the lateral first reflections, occurring at much larger angles, are also in reasonable condition (Figure 16.10). The bottom graphs show separate vertical and horizontal directivity indexes for these designs. The horizontal dispersion problem with the MTM layout and the virtue of the three-way design are apparent. Data provided by the creator of this informative display, William Decanio, Harman Consumer Group.
These designs also show up in vertical arrangements, in which case the acoustical interference is heard after reflection from the floor and ceiling. An intermediate configuration, sometimes called the 2½-way, rolls off one of the woofers at a low frequency, allowing the second unit to function as a midrange. The result is a slight improvement in overall performance, but the horizontal-plane interference pattern is then asymmetrical and still not what is needed. The real solution is to add a midrange loudspeaker allowing both woofers to be crossed over at a frequency sufficiently low that the acoustical interference is avoided. The explanation is in the caption.